The present invention relates to a hydraulic auxiliary drive system for driving the normally non-driven wheels of a vehicle. Conventional heavy duty industrial machinery such as road graders and the like include a pair of driven rear wheels and a pair of steerable normally non-driven front wheels. Under certain conditions, it is desirable to provide a variable torque drive to the normally non-driven wheels to enhance the tractive effort of the vehicle. The present invention relates to an improved hydraulic auxiliary drive system for driving such wheels which is readily operable between forward and reverse conditions and which includes means for protecting the drive motors of the system from an undesirable overspeed condition such as when the vehicle front wheels are off the ground or are on an icy surface affording little or no traction.